Coverage with temephos programme
Our three measures of household coverage (five or more visits from the temephos team in the last 12 months, temephos last placed in household water less than 3 months ago, and temephos observed in at least one water container during the survey) all indicate low coverage of the government temephos application programme in 2012, especially in rural areas. Only 6% of rural households said they had been visited five times or more and four out of ten said they had not been visited at all in the last year. The situation was slightly better in urban areas, where only two out of ten households had been missed altogether and 12% reported five visits or more. Whatever the number of programme visits across the year, less than half the households, at the beginning of the dry season, reported having temephos placed in their water containers within the last 3 months (that is, during the peak dengue season). And the field teams observed temephos (in any water container) in even fewer households: 30% in urban sites and 14% in rural sites. This is far from universal coverage, which was the aim of the programme in 2012.
We can compare our estimates of coverage with the government dengue control programme figures for January to December 2012 reporting the number of household visits made by the vector control teams in the three regions. Based on these figures, the census populations of the regions, and estimated household size, some 18% of households had three visits during the year in Costa Grande, 6% in Costa Chica, and 37% in Acapulco (Dr Rufino Silva Dominguez, Personal Communication; Additional file 1). From the government programme figures, across the three regions, about 26% of households had three visits during the year. This figure is quite close to the overall figure from the survey of temephos observed in at least one container in 20.6% of households. There are some differences between regions, with a relatively high proportion of households in Costa Chica having temephos observed in the survey, despite a lower number of visits reported by the government programme in this region. This might reflect that households in this region retain the applied temephos in their water containers for longer than in the other regions.
We found few associations between temephos coverage and socio-economic variables. The main factor was area of residence, with much higher coverage in urban areas. Within rural areas, smaller households were more likely to have temephos present, although such households have been found to have a higher risk of self-reported dengue cases, perhaps because of better recognition of the condition [32]. Temephos application in response to recognition of a clinical case of dengue might explain the association with household size. There is evidence that households without a regular water supply have higher entomological indices [33] and higher rates of dengue cases [34], and perhaps they should be especially targeted by the temephos programme, but we found no association between water supply and temephos coverage.
There is surprisingly little published evidence about coverage with routine government temephos application programmes. A cross-sectional survey of 966 households in Thailand reported higher use of temephos in rural (60%) than in urban households (25%) in the last 12 months; with 16% of rural households and 7% of urban households treating with temephos more often than quarterly [25]. This contrasts with our much higher coverage in urban areas. Unlike in Mexico, where government officers place temephos in household water containers, in Thailand, temephos is delivered to households and household members are responsible for placing the chemical in their water containers; the system of distributing temephos is different between urban and rural sites. Less than half (47%) of 2512 respondents to a telephone survey in Malaysia reported putting temephos in their water containers to prevent mosquitoes breeding there, while most (75%) reported covering their water containers [26].
The coverage of temephos application achieved can make a difference. A study in Clorinda, Argentina, of the impact of a programme of city-wide household inspections and temephos application found that coverage with temephos varied between areas and that reduction in larval indices was related to the proportion of households visited and treated [35].
Perceptions about temephos and mosquito control
About a quarter of household respondents in our study considered bathing in water containing temephos was, or might be, damaging to health, but the majority (69%) believed that drinking or cooking with water containing temephos posed, or might pose, a health risk. Other authors have reported public concerns about adverse health effects of temephos. A qualitative study in Thailand reported a common perception that temephos is a harmful chemical that should not be placed in drinking water [36]. Interviews with 54 people in Cuba living in houses where Aedes aegypti were detected suggested that some of them were convinced that water containing temephos was not safe to drink [37]. Focus groups and in-depth interviews in Peru suggested that discolouration and bad taste, rather than health concerns, were the main reason for refusing temephos addition to drinking water [38].
The great majority of household respondents in our study agreed that fumigation and temephos application was the best way to avoid mosquitoes. This belief may reflect the strong reliance on these methods by the government vertical programme for dengue control over many years. The government programme typically intensifies activities such as temephos application and fumigation at the height of the annual dengue season, under pressure from the media and communities themselves. From this point, the number of dengue cases and mosquitoes will reduce in any case, from reduction of susceptibles and with the advent of the dry season with less vector breeding, but the reduction is likely to be associated in people’s minds with the visible intensification of chemical-based actions. Other authors have reported strong public belief in the efficacy of temephos and, in particular fumigation, for mosquito control. A study in three communities in Mexico, using in-depth interviews, reported a prevailing belief in the government programme of using temephos and fumigation to control dengue, despite some concerns about frequency, safety, and efficacy of temephos application [39]. The authors considered that this could be related to the long-standing use of the same vertical programme for dengue prevention, or because knowledge and beliefs did not favour a change towards more individual responsibility for prevention. Focus groups in Thailand considered that insecticide spraying was the best way to control mosquitoes [36]. However, in Peru, some groups were reluctant to agree to fumigation because they thought it was ineffective [38].
Impact of Camino Verde intervention
In the Camino Verde trial, the government dengue control programme, including temephos application, continued in both intervention and control sites, and the intervention was not designed to detract from this programme [24]. However, we did detect a reduction in presence of temephos in intervention clusters compared with control clusters in the impact survey, perhaps related to the larger proportion of households removing applied temephos after only a short time. A greater proportion of households in intervention sites believed bathing with water containing temephos could be harmful. And there was a significant, small, reduction in the proportion of respondents who agreed temephos and fumigation was the best method to avoid mosquitoes, although this figure remained very high. Perhaps the intervention focus on non-chemical means of vector control meant that some people began to question the reliance on temephos and other chemicals. It is possible that in some clusters, members of the local mobilisation teams (brigadistas) may have specifically discouraged the use of temephos, while encouraging households to use alternative methods for preventing mosquito breeding. The materials used by the brigadistas concerned the mosquito life-cycle and how this could be interrupted by non-chemical means, but they did not specifically discourage the use of temephos. In some intervention communities it became popular to use larvivorous fish to reduce mosquito breeding [40]. These fish cannot survive in water containing temephos, and households with fish present in any water container were indeed much less likely to have temephos found in any water container (OR 0.26, 95% CIca 0.18–0.36).
But it is clear that, despite the intervention, most households continued to believe that a programme outside their own control is what is needed to control the dengue vector. The report of the Camino Verde trial noted that significantly more households in intervention clusters than control clusters believed that communities themselves could control dengue, but nevertheless this proportion was less than 50% even in intervention clusters [24].
Limitations
The estimate of temephos coverage relying on recall of visits over the last 12 months may not be entirely reliable, perhaps tending to under-estimate visits that happened longer ago. However, the low coverage reported by households was also reflected in figures from the government vector control programme. Recall of the timing of the last application of temephos is likely to be more reliable, and the direct observation of temephos in the water containers does not rely on recall. The question about the best method for mosquito control was not an open question and only one method was mentioned. The question was worded “Many people believe that the best method to avoid mosquitoes is to use temephos and fumigation. Do you agree this is the best method?” This approach might have inflated the apparent support for temephos and fumigation as the best method for mosquito control.